Study of Physicochemical Properties of Dried Kiwifruits Using the Natural Hypertonic Solution in Ultrasound-assisted Osmotic Dehydration as Pretreatment

ABSTRACT

In this study, the effect of osmotic solutions (sugar solution, grape syrup and mulberry syrup) and ultrasound time (20, 30, and 40 min) was investigated on the water loss, solid gain, moisture content, shrinkage, firmness, color (L*a*b*), vitamin C, total chlorophyll, and sensory properties of dried kiwifruits. High performance liquid chromatography revealed that grape and mulberry syrups contained high amounts of invert sugar as fructose and glucose. The results showed that the use of grape syrup as pre-treatment reduced the moisture content, shrinkage and L* value of the kiwifruit slices while increasing the water loss, solid gain, firmness, vitamin C and total chlorophyll of the samples compared to other pretreatments. Moreover, with increasing ultrasonic time from 20 min to 40 min, moisture content, vitamin C, and total chlorophyll decreased whereas firmness increased in the samples. The results also demonstrated that the use of grape and mulberry syrups significantly improved the organoleptic characteristics of the dried kiwifruit slices.

KEYWORDS:

• Grape syrup
• mulberry syrup
• ultrasonic
• drying
• fruit

Introduction

Kiwifruit (Actinidia deliciosa) contains vitamin C twice as much as oranges and six times as much as lemons. In addition, black seeds in kiwifruit are full of vitamin D (Zhao and Zhao, 2006). Kiwifruit includes vitamin A and the vitamin B group and also has a good antioxidant capacity due to a wide number of phytonutrients. Further, it has a high amount of magnesium and thus decreases the possibility of some types of cancers and heart disease. Generally, kiwifruit has a high amount of nutrients and a low amount of calorie and thus can be considered as a highly nutritious fruit (Salim et al., 2017; Traffano-Schiffo et al., 2017). Iran with 294413 tons of kiwifruit was the fourth kiwifruit producer in 2016, after China, Italy, and New Zealand (FAOSTAT, 2017). The major use of this product is when it is fresh. Due to its softness and low shelf life, drying is considered to be a significant method for preservation of kiwifruit (Chakraborty and Samanta, 2017).

The traditional drying methods, in addition to consuming much time and energy, make some changes in products such as tissue shrinkage with a rigid surface; unfavorable changes in color, taste, and aroma; and a drop in water absorption and nutrition values of products (Amami et al., 2017; Bromberger Soquetta et al., 2018). Therefore, choosing suitable drying and pretreatment methods will increase the quality of final product (Deng et al., 2019).

Osmotic dehydration is one of the common pretreatments before air-drying, which partially removes water from tissues of fruits or vegetables immersed in a hypertonic (osmotic) solution. In this way, the required force for mass transfer is provided by osmotic pressure generated across the cellular surface, which acts as an effective semi-permeable membrane (Hamedi et al., 2018; Nowacka et al., 2014; Sette et al., 2015; Zhao et al., 2014).

Studies have indicated that using a combined osmotic and ultrasonic pretreatment is a cheap and simple method to reduce drying duration and processing cost while increasing product quality (Garcia-Noguera et al., 2010; Goula et al., 2017; Nowacka et al., 2018).

Ultrasonic waves will cause dehydration or other single operations in the drying process with variable mechanisms, such as increasing temperature in the boundary layer, pressure changes due to cavitation, micro-channel development with crack creation caused by shear stress of cavitation, boundary layer confusion, and structural changes of the surrounding environment. Variable sound stresses facilitate the drying process via preserving the existing channels or creating new ones. Pretreatments are carried out by using ultrasonic waves and immersing materials in water or hypertonic solution. Researches have also revealed that changes in ultrasonic waves are far less in samples immersed in water than those immersed in sucrose solution (Fernandes and Rodrigues, 2008). Moreover, samples immersed in water consume a shorter drying time in comparison with those immersed in ultrasonic osmotic solution (Amami et al., 2017; Bromberger Soquetta et al., 2018; Wang et al., 2019).

Sucrose solution is the most popular hypertonic solution. Moreover, with the ever-increasing demand for production and consumption of high-nutritious food, a potential alternative appears to be the use of other media, e.g. concentrated fruit juice. Bchir et al. (2012) used date juice with sucrose as immersion solution to the osmo-dehydration of pomegranate seeds and showed that it not only decreased the process cost by reducing the amount of sucrose added to the osmotic solution but also improved the nutritional value of the product with high natural sugar content (Bchir et al., 2012).

Grape and mulberry are rich sources of various nutrient components, such as vitamins, minerals, carbohydrates, edible fibers and antioxidants that are useful for human health (Singhal et al., 2010; Xia et al., 2010). Grape and mulberry syrups called pekmez in Turkish are a kind of fruit products, which have been traditionally manufactured since ancient times in most Iranian regions by boiling varieties of grape and mulberry juice up to about 70% soluble dry matter concentration without sugar or other additives (Mohamadi Sani, 2013; Shahidi et al., 2017; Turkben et al., 2016). These syrups contain high amounts of sugar, mineral and organic acid, which are nutritionally important for infants, children, and sportsmen, as well as in situations demanding urgent energy supply. The reason for the latter is that grape and mulberry syrups easily pass into the blood without digestion because most of their carbohydrate is glucose and fructose (Mohamadi Sani, 2013; Sengül et al., 2005). These products are fabricated due to the presence of monosaccharides as a natural sweetener and a suitable replacement for sucrose in the food industry (Sarabandi et al., 2014; Singhal et al., 2010). Iran is the source of black mulberry, which is one of the most important types of mulberry growing throughout the country (Kamiloglu et al., 2013), although there is no official information on its annual production. Moreover, Iran was the ninth largest producer of grapes with an annual production of close to 2.45 million tons in 2016 (FAOSTAT, F., 2017).

To our knowledge, there has been no study in the literature so far devoted to the use of natural syrup for preparing osmotic solution to assist ultrasound dehydration as a pretreatment in the air-drying process of kiwifruit. Therefore, the aim of this research was to investigate the effect of using grape and mulberry syrups as hypertonic solutions in ultrasound assisted osmotic dehydration as a pretreatment dehydration of kiwifruit slices.

Materials and Methods

Sample Preparation

Grape and mulberry syrups were provided from the local market of Borujerd (Lorestan, Iran). Moreover, kiwifruits were purchased from the local market of Mollasani (Khuzestan, Iran) and kept at the temperature of 4 to 6°C before the drying process. The fruits were taken out of the refrigerator 2 hours before the process and put at room temperature. The intact fruits, which had the similar size and color, were chosen as study samples. The fruits were washed and cut in 0.6 cm thick slices. In order to prevent the browning reactions, the samples were immersed in 1% citric acid for 5 min. Then, they were washed in distilled water and dried using absorbent paper so that their weight could be estimated. Afterward, the samples were pretreated. The moisture content was dried by putting the samples in an oven (Binder WTC, Tutlingen, Germany) at 100°C in order to reach the constant weight.

Ultrasound-assisted Osmotic Dehydration Pretreatment

Three osmotic solutions (sugar solution, grape syrup, and mulberry syrup) were produced using 55 °Brix. The samples were added to the beaker including the osmotic solutions and distilled water in a ratio of 1 to 10 (the ratio of fruit weight to solution volume). Then, the beaker was put into the ultrasonic bath (TransSonic TP 690-A, Elma, Germany) at 27 KHz and 65°C in three time periods of 20, 30, and 40 min. The brix of the osmotic solutions, the ratio of fruit weight to solution volume and the temperature were selected based on initial experiments.

Final Drying

After the treatment, the samples were removed from the osmotic solution and washed by distilled water and then their surface moisture was dried by absorbent paper. The final drying of the samples was performed by a cabinet dryer machine (Model X₁, Behdooneh Co., Babolsar, Iran) at 50°C in order to reach 20% moisture content.

Physicochemical Properties of Syrups

Total soluble solids of the grape and mulberry syrups were determined as °Brix using a digital refractometer (Atago Ltd., Tokyo, Japan). Moreover, ash, protein, titratable acidity and pH were measured according to the AOAC official method (AOAC., 2000).

The high performance liquid chromatography (HPLC) method was applied to analyze the sugar content of the grape and mulberry syrups including sucrose, fructose and glucose. The samples were diluted with deionized water and then 20 µL of each sample after filtration through a 0.45 μm Millipore filter was injected to an HPLC apparatus equipped with a Eurokat H column and a smart line RI detector 2300 (Knauer, Berlin, Germany). The mobile phase was 0.005 M sulfuric acid with a flow rate of 0.8 ml/min at 70°C. Eventually, the sugars were identified according to the retention time of the standards. In addition, the color of the grape and mulberry syrups was investigated by measuring L* (lightness), a* (green-red) and b* (blue-yellow) parameters using a Minolta colorimeter CR-400 (Konica Minolta, Inc., Osaka, Japan).

Moisture Content

The moisture content was estimated by accurately weighing the ground samples after oven drying (Binder WTC, Tutlingen, Germany) them at 105°C until reaching the constant weight according to the AOAC official method (925.40), which was calculated based on the initial and final sample weights (AOAC, 2000).

Water Loss (WL) and Soluble Solid Gain (SG)

WL and soluble SG were determined using the weight of the kiwifruits before and after the pretreatment trials. In addition, the moisture content (wet basis) of the kiwifruits was determined before and after the pretreatment. The WL and SG of the samples were evaluated using the following equations (Horuz et al., 2017):

(1) $\mathrm{W}\mathrm{L}=\frac{{\mathrm{W}}_{\mathrm{i}}{\mathrm{X}}_{\mathrm{i}}-{\mathrm{W}}_{\mathrm{f}}{\mathrm{X}}_{\mathrm{f}}}{{\mathrm{w}}_{\mathrm{i}}}$(1)

(2) $\mathrm{S}\mathrm{G}=\frac{{\mathrm{W}}_{\mathrm{f}}\mathrm{X}{\mathrm{s}}_{\mathrm{f}}-{\mathrm{W}}_{\mathrm{i}}\mathrm{X}{\mathrm{s}}_{\mathrm{i}}}{{\mathrm{W}}_{\mathrm{i}}}$(2)

where w_i is the initial fruit mass (g) before the pretreatment; w_f is the final fruit mass (g) after the pretreatment; X_i is the initial fruit moisture content on a wet basis (g water/g total fruit mass) before the pretreatment; X_f is the final fruit moisture content on a wet basis (g water/g total fruit mass) after the pretreatment; X_si is the initial fruit dry solid matter content (g dry matter/g total fruit mass) before the pretreatment; and X_sf is the final fruit dry matter content (g dry matter/g total fruit mass) after the pretreatment.

Shrinkage

The shrinkage caused by the different dehydration methods was calculated through measurements of sample volume change. Volume was estimated gravimetrically by displacement of toluene in a pycnometer based on the method described by Sette et al. (2015).

Color

The color changes of the fresh and dehydrated kiwifruit slices were measured with a Minolta colorimeter CR-400 (Konica Minolta, Inc., Osaka, Japan). This spectrophotometer uses an illuminant D₆₅ and a 10° observer as references. Color data are provided as CIE L*a*b* coordinates, which defines color in a three dimensional space. L* indicates lightness, taking values within the range between 0 (black)–100 (white), and a* and b* are green-red and blue-yellow coordinates, respectively. Moreover, a* takes positive values for reddish colors and negative values for greenish ones whereas b* takes positive values for yellowish colors and negative values for bluish ones.

Firmness

The firmness of the samples was evaluated by performing a penetration test using a TA-XT plus texture analyzer (Stable Micro Systems, Surrey, UK) equipped with a 25 N load cell. The experiment was run with a stainless steel probe of 6 mm diameter, with of 1 mm/s. The result was reported as firmness in Newton (N) as a maximum force (N) necessary to penetrate the kiwifruit slices (Hamedi et al., 2018).

Vitamin C

The vitamin C content of the kiwifruit slices was determined using 2,6 – Dichloroindophenol Titration method, as described in the Official Method of Analysis, Method 967.21. (AOAC, 2000). (Huang et al., 2017; Kabasakalis, 2000)

Total Chlorophyll Content

A spectrophotometric method was used to evaluate total chlorophyll content (Nowacka et al., 2017). Briefly, after weighing 250 mg of the samples, 5 ml of 80% (v/v) acetone solution was added. The mixture was kept in dark for 15 min. Then, for 10 min, the samples were centrifuged at 1500 g. The 80% (V/V) acetone was used to make the filtrate. The absorbance at 663 (A₆₆₃) and 645 (A₆₄₅) nm was evaluated using spectrophotometry (Cary 100 Bio Varian, Palo Alto, USA). The following equation was used to measure the concentration:

(3) ${\mathrm{C}}_{\mathrm{a}}=12.21\left({\mathrm{A}}_{663}\right)-2.81\left({\mathrm{A}}_{646}\right)$(3)

(4) ${\mathrm{C}}_{\mathrm{b}}=20.13\left({\mathrm{A}}_{646}\right)-5.03\left({\mathrm{A}}_{663}\right)$(4)

(5) ${\mathrm{C}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}=\left(1000\times {\mathrm{A}}_{470}-3.27{\mathrm{C}}_{\mathrm{a}}-104{\mathrm{C}}_{\mathrm{b}}\right)/229$(5)

Sensory Evaluation

The organoleptic characteristics of the dried kiwifruit slices including taste, appearance, texture, and overall acceptability were evaluated by a 10-member trained panel at ages 22–28 (six female and four male) with experience in sensory evaluation of foods. The measurements were performed in individual booths with controlled illumination and temperature. The panelists were asked to indicate how much they liked or disliked each sample on a 5-point hedonic scale (5 = like extremely; 1 = dislike extremely) according to overall acceptance. The entire experiment was repeated three times, and the sensory scores were presented as the overall mean.

Statistical Analysis

The data reported in this study is the mean of a minimum of three replicates. All the data were subjected to analysis of variance (ANOVA) and later to the Duncan’s multiple range test to evaluate significant differences among the treatments at p = .05. Statistical analyses were performed using SPSS 23.0 (SPSS Science, Chicago, IL, USA).

Results and Discussion

Physicochemical Properties of Syrups

Some physical and chemical properties of the grape and mulberry syrups are given in Table 1. The chemical values of the grape and mulberry syrups were almost close to each other. As observed in Table 1, soluble dry matter, total sugar, ash, pH, protein, and titratable acidity were calculated to be 71.91 (%), 60.77 (g/100 g), 2.01 (%), 5.11, 1.81 (%), and 0.57 (%) for the grape syrup and 72.23 (%), 58.51 (g/100 g), 2.24 (%), 5.03, 1.94 (%), and 0.61 (%) for the mulberry syrup, respectively. As also shown in Table 1, the examined syrups contained high amount of total sugar, which was measured to be 60.57 and 58.71 (g/100 g) for the grape and mulberry syrups, respectively. In addition, the invert sugars, mainly, fructose and glucose, were found to be the major constituents of the grape and mulberry syrups, while the syrups contained an extremely small amount of sucrose. These results are in agreement with Turkben et al. (2016) and Sengül et al. (2005), who studied the characteristics of pekmez produced from grape and mulberry, respectively. However, caramelization of sugars or Maillard reactions were the non-enzymatic browning reactions that caused the color of the syrups to darken and thus the syrups did not have high lightness (L*) index, which confirms the reports of Sengül et al. (2005).

Table 1. The physicochemical properties of the grape and mulberry syrups

Parameters	Grape syrup	Mulberry syrup
Soluble dry matter (%)	71.91 ± 2.31^a	72.23 ± 2.85 ^a
Total sugar (g/100 g)	60.57 ± 3.76 ^a	58.51 ± 3.93 ^a
Fructose (g/100 g)	29.65 ± 1.84 ^a	29.05 ± 2.06 ^a
Glucose (g/100 g)	30.08 ± 1.75 ^a	28.91 ± 1.77 ^a
Sucrose (g/100 g)	0.84 ± 0.17 ^a	0.55 ± 0.10 ^a
Ash (%)	2.01 ± 0.13 ^a	2.24 ± 0.15 ^a
pH	5.11 ± 0.19 ^a	5.03 ± 0.23 ^a
Protein (%)	1.81 ± 0.48 ^a	1.94 ± 0.66 ^a
Titratable acidity (% citric acid)	0.57 ± 0.08 ^a	0.61 ± 0.10 ^a
L*	23.64 ± 3.17 ^a	27.09 ± 2.36 ^a
a*	8.22 ± 1.69 ^a	7.04 ± 2.01 ^a
b*	1.07 ± 0.82 ^a	3.26 ± 1.02^b

Means followed by the same letters in rows are not significantly different (p < .05).

Moisture Content

The moisture content of the treated samples is indicated in Table 2. Based on the results, type of osmotic solution and ultrasonic time affected the moisture content of the samples significantly (P < .05). Accordingly, the amount of moisture content decreased with increasing ultrasonic time. The lowest amount of moisture content was observed in samples immersed in the grape and mulberry syrups in the entire ultrasound period. This difference was caused by higher amount of monosaccharide (glucose and fructose) in the grape and mulberry syrups, which created a higher osmotic pressure (Shahidi et al., 2012). The osmotic drying process with a longer duration will change the absorption characteristic of the cell wall and cause more WL (Zhang et al., 2017).

Table 2. The effect of type of osmotic solution and ultrasonic time on the moisture content (%), water loss (%), solid gain (%), shrinkage (%), vitamin C (mg/100 g), and total chlorophyll (mg/kg) of the kiwifruit slices during the osmotic-ultrasound-assisted osmotic dehydration pretreatment

		Moisture (%)				Water loss (%)
Ultrasound time(min)	Mulberry syrup	Grape syrup	Sugar	Distilled water	Mulberry syrup	Grape syrup	Sugar	Distilled water
20	45.87 ± 1.80^Caa	42.25 ± 1.38^Ca	54.38 ± 2.95^Cb	67.19 ± 1.33^Cc	14.50 ± 2.31^Ab	17.13 ± 1.48^Ab	8.08 ± 0.72^Aa	7.74 ± 1.71^Aa
30	32.34 ± 2.23 ^Ba	28.22 ± 2.52 ^Ba	43.43 ± 2.27^Bb	61.88 ± 1.87^Bc	18.54 ± 2.25^Ab	20.21 ± 2.65^Ab	9.06 ± 2.19^Aa	7.92 ± 1.13^Aa
40	22.41 ± 2.24^Aa	20.76 ± 1.81^Aa	39.90 ± 1.44^Ab	49.58 ± 2.45^Ac	36.83 ± 4.75^Bb	37.20 ± 3.42^Bb	11.58 ± 1.43 ^Ba	9.03 ± 1.91 ^Ba
		Solid gain (%)				Shrinkage (%)
20	6.41 ± 1.44^Ac	7.33 ± 1.58^Ac	2.13 ±.94^Ab	1.40 ±.29^Aa_	36.83 ± 3.09^Ab	31.88 ± 1.78^Aa	46.83 ± 2.55^Ab	50.93 ± 2.40^Ac
30	6.52 ± 1.54^Ab	7.58 ± 1.66^Ab	3.40 ±.91 ^Ba	2.78 ± 1.23^Ba_	51.63 ± 1.61^Bb	46.33 ± 1.56 ^Ba	63.93 ± 2.57^Bb	68.93 ± 1.85^Bc
40	9.70 ± 1.68^Bb	9.87 ± 1.64^Bb	3.64 ±.84 ^Ba	2.85 ± 1.25^Ba_	58.67 ± 2.45^Cb	48.52 ± 2.46 ^Ba	78.27 ± 2.07^Cc	81.66 ± 1.56^Cc
		Vitamin C(mg/100 g)				Total chlorophyll (mg/kg)
20	56.90 ± 1.88^Cc	69.20 ± 1.95^Bd	45.17 ± 1.47^Cb	31.16 ± 2.15^Ca	0.36 ± 0.10 ^Ba	0.56 ± 0.08^Ab	0.32 ± 0.07^Ca	0.34 ± 0.15^Ca
30	48.60 ± 1.68^Bc	58.20 ± 1.98^Ad	36.96 ± 1.94^Bb	28.94 ± 1.38 ^Ba	0.23 ± 0.09^Bb	0.48 ± 0.10^Ac	0.21 ± 0.16^Bb	0.17 ± 0.01 ^Ba
40	38.13 ± 1.74^Ac	56.90 ± 2.14^Ad	30.50 ± 1.73^Ab	19.67 ± 1.67^Aa	0.17 ± 0.01^Ab	0.29 ± 0.12^Ac	0.14 ± 0.02^Aa	0.11 ± 0.10^Aa

^aMeans followed by the same letters, lower case in rows and capital in columns, are not significantly different (p < .05).

Water Loss (WL)

The results of WL after the treatments are presented in Table 2. As shown, using the natural osmotic solutions (the grape and mulberry syrups) caused a significant increase (P < .05) in the amount of WL in the samples. This issue is probably caused by monosaccharide such as glucose and fructose in natural osmotic solutions (grape and mulberry syrups). Monosaccharide has smaller molecular size and weight in comparison with sucrose. Therefore, it creates higher osmotic pressure and causes more WL in the samples (Panagiotou et al., 1999). Using grape syrup as the osmotic solution causes more WL during the drying process in comparison with other treatments. This is due to the higher amount of glucose and fructose in grape syrup, which causes higher WL in the samples during the drying process. This report is consistent with the reported results of Zhang et al. (2017), who researched the osmotic drying of peach (Zhang et al., 2017). Furthermore, with the increase of ultrasonic time, the amount of WL also increased in the samples. The reason is that with the passage of time, the ultrasonic waves made more pores and channels in the structure of the samples. Therefore, it can be expected that this phenomenon accelerated the speed of WL during the osmotic drying process (Amami et al., 2017).

Solid Gain (SG)

The results of variance analysis (Table 2) revealed that using the osmotic solutions had a significant influence (p < .05) on the amount of SG of the samples. Accordingly, using the grape and mulberry syrup instead of the sugar solution increased (p < .05) the amount of SG of the samples. Using osmotic solutions with low molecular weight (such as grape and mulberry syrup) compared to osmotic solutions with high molecular weight (such as sugar solution) makes the absorption process easier. In this regard, fructose causes more osmotic pressure in tissues since it has a higher ability to bond with water (Panagiotou et al., 1999). Therefore, After drying moisture, sugary compounds are absorbed into tissues. In this case, the amount of SG increases.

Shrinkage

Changes in shrinkage of the dried kiwifruit slices are presented in Table 2. As observed in Table 2, the increase of ultrasound time increased the amount of shrinkage in all the samples. In addition, the shrinkage of the samples decreased after using the osmotic solution and the most reduction was observed in dried kiwifruit slices pretreated by the grape syrup before drying, since the most shrinkage was observed in samples which immersed in distilled water. In fact, the absorbed solids filled the gaps existing in the samples, which, in turn, prevented shrinkage (Fathi et al., 2011). Since glucose and fructose in grape syrup have lower molecular weight in comparison with sucrose in other treatments, their absorption causes a significant reduction in shrinkage (Shahidi et al., 2012). Shahidi et al. (2012) evaluated osmotic and ultrasonic pretreatments in dried bananas. They declared that type of osmotic solution, time, and solution concentration influenced WL and shrinkage. Furthermore, the most amount of WL and the least amount of shrinkage were observed in the 50% glucose solution (Shahidi et al., 2012).

Vitamin C

As shown in Table 2, type of osmotic solution and ultrasonic time had a significant effect on vitamin C (P < .05). Accordingly, longer ultrasonic time decreased the amount of vitamin C in the samples. This is probably due to the formation of micro-channels during the cavitation process. The water-soluble nature of vitamin C causes more vitamin C loss in the samples (Egea et al., 2012; Vissers et al., 2013). Furthermore, using the natural osmotic solutions (the grape and mulberry syrup) instead of the sugar solution and distilled water increased the amount of vitamin C (P < .05). This behavior was related to the protective effect of sugar on vitamin C; the grape syrup indicated the highest protection effect in the kiwifruit samples. So that the amount of vitamin C in the raw kiwifruit samples (94.24 ± 1.5 mg/100 g) was reduced to 56.9 ± 2.14 (mg/100 g) in the samples treated with the grape syrup.

Total Chlorophyll

The effects of type of osmotic solution and ultrasonic time on total chlorophyll are indicated in Table 2. Based on the results, higher ultrasonic time decreased the amount of chlorophyll in the samples. Using the ultrasonic process causes tissue damage and leakage of biologically active compounds such as chlorophyll into the osmotic solution. Furthermore, during the ultrasonic process, an active form of oxygen is produced, which consequently creates a cavitation phenomenon. This process removes biologically active compounds (e.g., chlorophyll). Therefore, longer ultrasonic time will decrease the amount of total chlorophyll. Enzymatic and non-enzymatic reactions will break color pigments and chlorophylls. Using the osmotic solutions instead of distilled water increased the amount of chlorophyll. The reason is that the kiwifruit slices treated with the grape syrup as the osmotic solution, prior to drying, were found to have the highest chlorophyll. This increase was more significant when using the grape and mulberry syrup as the osmotic solutions (P < .05). This was probably due to the increase of the aqueous phase viscosity, affecting the reduction diffusion of chlorophylls.

Firmness

The results of experiment on the firmness of the kiwifruits are illustrated in Figure 1. As can be observed in Figure 1, type of treatment and ultrasonic time affected the firmness of the samples (P < .05). Accordingly, increasing ultrasonic time increased firmness. In other words, increasing ultrasound time reduced the texture rigidity and made it softer and more elastic. In addition, using the osmotic solutions instead of distilled water increased firmness, which was more significant when using the grape syrup. The firmness of the samples while using the osmotic solutions is probably due to the increase of solid matter and decrease of moisture content (Nieto et al., 2013). Grape and mulberry syrups as osmotic solutions have higher osmotic pressure and thus they increase the amount of solid matter while decreasing moisture content. In this way, the samples become firmer (Nowacka et al., 2018).

Figure 1. The effect of type of osmotic solution and ultrasonic time on the firmness of the dried kiwifruit slices

Color

The colorimetric indexes of different types of treatment are shown in Table 3. As can be observed from Table 3, significant differences were detected between the samples treated with distilled water and those treated with other methods in terms of the L*, a*, and b* values (P < .05). Furthermore, using the osmotic solutions (i.e., the sugar solution, the grape syrup and the mulberry syrup) instead of distilled water changed the colorimetric indexes of the samples during the process (P < .05). Accordingly, the samples treated with the grape and mulberry syrups had the most colorimetric changes in the color index due to the increased temperature and monosaccharide absorption (glucose and fructose). Grape and mulberry syrups as osmotic solutions contain more monosaccharide (glucose and fructose); therefore, they cause more significant color index changes. Changes in the color index during the process are probably influenced by the colored pigments of osmotic solutions (grape and mulberry syrups), pigmented chemical interactions such as Milliard’s reaction between sugar and protein and production of Melanoidin. Based on the chemical structure of glucose and fructose, these types of sugar have a more active role in interactions and production of color pigments (Aadil et al., 2013; Wiktor et al., 2016).

Table 3. The effect of type of osmotic solution and ultrasonic time on the color characteristics of the dried kiwifruit slices

	L* value				a* value				b* value
Ultrasound time(min)	Mulberry syrup	Grapesyrup	Sugar	Distilledwater	Mulberrysyrup	Grapesyrup	Sugar	Distilledwater	Mulberry syrup	Grapesyrup	Sugar	Distilled water
20	40.36 ± 0.92^Ab	41.52 ± 0.53^Ab	49.33 ± 0.98^Bc	42.29 ± 0.67^Aa	−1.92 ± 0.26^Ab	1.50 ± 0.24^Ac	−4.62 ± 0.28 ^Ba	−4.85 ± 0.41 ^Ba	16.28 ± 0.66^Bc	9.56 ± 0.78^Aa	13.77 ± 0.65^Cb	16.52 ± 0.20^Bc
30	39.17 ± 0.91^Bb	38.99 ± 0.71^Bb	42.41 ± 0.91^Bc	35.78 ± 0.95^Aa	−1.63 ± 0.06^Ac	2.18 ± 0.04^Ad	−4.03 ± 0.04 ^Ba	−2.98 ± 0.14^Ab	14.98 ± 0.98^Bb	9.33 ± 0.70^Aa	10.65 ± 0.84 ^Ba	9.96 ± 0.53^Aa
40	25.28 ± 0.67^Aa	24.35 ± 0.79^Aa	21.56 ± 0.79^Aa	28.86 ± 0.93^Bb	−1.13 ± 0.01^Ab	2.54 ± 0.06^Bc	−2.60 ± 0.12^Aa	−2.76 ± 0.31^Aa	11.83 ± 0.83^Ab	9.11 ± 0.12^Aa	7.50 ± 0.08^Aa	8.33 ± 0.42^Aa

Means followed by the same letters, lower case in rows and capital in columns, are not significantly different (p < .05).

Sensory Evaluation

Figure 2 illustrates the mean sensory ratings of the dried kiwifruit slices. As can be observed in Figure 2, using the osmotic solutions instead of distilled water along with ultrasound time affected the quality parameters (P < .05), including appearance, texture, taste, and overall acceptability, of the dried kiwifruit slices. This study indicated that with increasing ultrasound time, the organoleptic characteristics of the dried kiwifruits were reduced. Moreover, the results showed that the appearance and texture of the samples significantly decreased in quality with increasing ultrasound time from 20 min to 40 min. Further, the best taste, texture, appearance, and overall acceptability were observed in the samples treated with the grape and mulberry syrups. This is probably under the influence of taste and color of the grape and mulberry syrups.

Figure 2. The effect of type of osmotic solution and ultrasonic time on the sensory properties of the dried kiwifruit slices

Conclusion

This study investigated the production of osmotic-dehydrated kiwifruit slices with natural osmotic solutions (grape and mulberry syrups) using ultrasound technology. The effect of operation parameters such as type of osmotic solution (sugar solution, grape syrup, mulberry syrup and distilled water) and ultrasound time (20, 30 and 40 min) was studied on the physicochemical properties of the samples. The results showed that moisture content was reduced while firmness, WL and SG increased in the samples with increasing ultrasound time and using the grape and mulberry syrups as the osmotic solutions. In addition, the remaining vitamin C and total chlorophyll of the dried kiwifruits increased with the use of the grape and mulberry syrups as the osmotic solutions, while the amount of shrinkage decreased in the samples. The findings of this work also showed that the use of the grape and mulberry syrups improved the sensory features of the kiwifruit slices. The results also indicated that using the grape syrup as the ultrasound osmotic pretreatment could improve the total quality of the dried kiwifruit slices. Since grape and mulberry syrups are cheaper than sugar solution and have extraordinary nutritional values, they can be considered as potential good immersion solutions for the osmotic dehydration of kiwifruit slices. In general, the use of grape and mulberry syrups instead of sugar solution as the osmotic solution in ultrasonic pretreatment used in fruit drying can be critical because of the improvement in the nutritional value of final product as well as reduction of drying costs.

Additional information

Funding

The authors acknowledge the financial support provided by Agricultural Sciences and Natural Resources University of Khuzestan.